Because of the large volume of business and residential telephone traffic that it carries, the PSTN is probably the most familiar of the traffic networks. This network provides the public switched telephone network services. The various types of traffic in the PSTN represent communications between any two end points in the network. Traffic is switched through each switching office, or node, it encounters and travels between nodes on trunk groups. The offices and trunk groups are arranged in a hierarchical routing structure.


To introduce the rudimentary operation of the network, this section presents a functional description of a typical telephone call, the most familiar service provided by the PSTN. The description illustrates some of the terms defined in previous sections and introduces some new terms and concepts.


Mrs. Cooper, a local realtor, is calling Mrs. Mahon, a prospective buyer, at her home in a neighboring town. Mrs. Cooper’s telephone is served by central office A, and her central office code is 747. Mrs. Mahon’s telephone is served by central office code 951 in central office B. Since many calls are placed between central offices A and B, a number of trunks provide a direct route between the two offices. An alternate route through tandem office C is also available.


When Mrs. Cooper picks up her handset, the switchhook contacts of the telephone set close, signaling its off-hook status. Control equipment in the switching system at office A detects a change from on-hook to off-hook status and interprets the change as a request for service. At this time, dial tone is connected to Mrs. Cooper’s telephone, assuming that a register, usually called an originating register is available to accept and e the digits she will dial. After Mrs. Cooper dials the first digit, the dial tone is disconnected. digits dialed by Mrs. Cooper (951-1234) are received and stored in originating register.



Next, the control equipment in central office A translates the dialed number. By examining the leading digits, usually the first three, it determines that Mrs. Cooper’s call is to another central office code; that is, it is not an intraoffice call. Her call is an interoffice call and must be connected to a trunk going to another office. Routing information stored in the system indicates which paths (trunk groups) are appropriate and translates the desired paths to representations of physical locations or terminations of trunks. If the call is billable, an automatic message accounting (AMA) register is requested. At this time, control equipment transfers the call information to a register in another storage area (the outpulsing register), releasing the originating register from the call. The control equipment begins scanning the outgoing trunks to find an idle trunk to office B. An idle trunk is found directly between offices A and B. The control equipment could have found that all trunks in the trunk group’s) to office B were busy. In this case, it would have begun to scan the outgoing trunks to tandem switching office C, since the call could be routed on a trunk from office A to office C and from there to office B. If all trunks to tandem office C had also been busy, would have been impossible to complete the call. In that case, Mrs. cooper would have heard a reorder tone, often called a fast busy tone since it has 120 interruptions per minute (ipm), compared to the 60 ipm if the busy tone.


The first event is the seizing of an idle trunk to office B. When a trunk is seized for a particular call, it appears busy to the switching system and becomes unavailable for other calls. A equipment in Mrs. Coopers central office will periodically scan for this ready signal.

When this ready signal is detected, outpulsing of digits begins. If central office B contains a single central office code, only the last four digits of Mrs. Mahon’s number will be transferred. This is because all calls on the direct trunk group will terminate at central office B. However, if office B contains more than one central office code additional digits must be transmitted to identify the particular central office code serving Mrs. Mahon.

Before the last digit is sent, the control equipment checks to see that the calling customer’s line is still off-hook. If the calling customer has hung up (abandoned the call), the control equipment will terminate the call-processing sequence and release associated equipment and circuits. When the last digit is outpulsed, the outpulsing register is released. The digits are now stored in the incoming register at central office B.


Once the digits are stored in an incoming register at the terminating office, many functions are initiated and supervised by the control equipment. The 4-digit line number is translated to Mrs. Mahon’s physical line termination. The status of Mrs. Mahon’s line is interpreted and signifies that the line is idle. (If Mrs. Mahon’s line were busy, a busy signal would be returned to Mrs. Cooper.)

The incoming trunk is connected through the switching network to Mrs. Mahon’s line. A ringing register is seized, the incoming register is released from this call, and Mrs. Mahon’s telephone rings. An audible ring, a tone that has the timing of a ringing signal and that indicates that a ringing signal is being applied to Mrs. Mahon’s telephone, is sent back to Mrs. Cooper at this time. The control equipment at the terminating office will scan Mrs. Mahon’s line status for an answer (off-hook) indication and, when it is detected, will terminate the ringing signal and return answer supervision to office A.

This will be used to record answer or connect time for billable calls. Mrs. Mahon answers the phone, and the conversation begins. As Mrs. Cooper talks into her handset, the acoustic speech signal is converted into an electrical signal by the transmitter in the handset. The signal generated by conventional transmitters is an electrical analog of the acoustic signal. This electrical analog of the speech may proceed through the switching systems and transmission facilities to Mrs. Mahon’s telephone in that form, or it may proceed through part of its path in digital form.

The latter would then require analog-to-digital and digital-to-analog conversions. With conventional technology, the signal reaching Mrs. Mahon’s telephone will be analog, and the receiver will convert the analog signal back to an acoustic signal. The acoustic signal from the receiver is not an exact reproduction of that at the transmitter. One reason for this is that the frequency content is limited by the transmission path. Also, impairments such as noise and loss occur, and if the call travels a long distance, an echo effect could occur.

During the conversation, the originating office, office A, monitors the outgoing trunk to office B for disconnect. If the calling party hangs up first, the connection is released, and disconnect supervision is sent to the terminating office. The trunk is idled when the terminating office returns on-hook supervision. If the called party (Mrs. Mahon, in this example) hangs up first, a timed-release period of 10 to 11 seconds is initiated.

The connection is released after this time—or earlier if the calling party hangs up. Completion of the call is detected and recorded at central office A for accounting purposes if there is a charge for the call; that is, if it is not covered by a fixed monthly charge or a flat rate. When the call is first dialed, the control equipment in central office A determines whether the call is billable by the routing information associated with the first three digits.

If the call is billable, a register is requested from an automatic message accounting system to receive information that is to be recorded about the call. For Mrs. Cooper’s call, the information recorded includes the number of Mrs. Cooper’s telephone, the number dialed, the time Mrs. Mahon answered, and the time the connection was released.

Data on this call and other billed calls from central office A are forwarded to a data-processing accounting center where they are periodically processed to compute customer charges. If the call is billable, Mrs. Cooper’s next monthly telephone bill will include a charge for the call. Thus, a basic telecommunications service—the simple telephone call—requires a relatively complex sequence of events.